The end of “walled garden” social networks?

I worry about attempting to build communities in walled gardens.

Is OpenSocial going to change the world? (If you need a quick intro to OpenSocial then watch this short video, or if your prefer reading try the API site).

Now all we need is a set of open source social network tools that allow us to build the niche social networks we need for real work whilst not expecting the participants to be totally cut off from the larger (and some would say more fun) networks that are currently popular.